The Challenge of Long-Term Climate Change
K. Hasselmann, et al.
Science 302, 1923 (2003);
DOI: 10.1126/science.1090858
The following resources related to this article are available online at
www.sciencemag.org (this information is current as of January 11, 2008 ):
Supporting Online Material can be found at:
http://www.sciencemag.org/cgi/content/full/302/5652/1923/DC1
A list of selected additional articles on the Science Web sites related to this article can be
found at:
http://www.sciencemag.org/cgi/content/full/302/5652/1923#related-content
This article cites 16 articles, 4 of which can be accessed for free:
http://www.sciencemag.org/cgi/content/full/302/5652/1923#otherarticles
This article has been cited by 18 article(s) on the ISI Web of Science.
This article appears in the following subject collections:
Atmospheric Science
http://www.sciencemag.org/cgi/collection/atmos
Information about obtaining reprints of this article or about obtaining permission to reproduce
this article in whole or in part can be found at:
http://www.sciencemag.org/about/permissions.dtl
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the
American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright
2003 by the American Association for the Advancement of Science; all rights reserved. The title Science is a
registered trademark of AAAS.
Downloaded from www.sciencemag.org on January 11, 2008
Updated information and services, including high-resolution figures, can be found in the online
version of this article at:
http://www.sciencemag.org/cgi/content/full/302/5652/1923
TRAGEDY OF THE COMMONS?
K. Hasselmann,1,2* M. Latif,3 G. Hooss,4 C. Azar,5 O. Edenhofer,1,6 C. C. Jaeger,1,6 O. M. Johannessen,1,7
C. Kemfert,1,4 M. Welp,1,6 A. Wokaun1,8
Climate policy needs to address the multidecadal to centennial time scale of climate
change. Although the realization of short-term targets is an important first step, to
be effective climate policies need to be conceived as long-term programs that will
achieve a gradual transition to an essentially emission-free economy on the time
scale of a century. This requires a considerably broader spectrum of policy measures
than the primarily market-based instruments invoked for shorter term mitigation
policies. A successful climate policy must consist of a dual approach focusing on both
short-term targets and long-term goals.
There is widespread consensus in the climate
research community that human activities are
changing the climate through the release of
greenhouse gases, particularly CO2, into the
atmosphere (1, 2). Because of the considerable inertia of the climate system— caused by
the long residence times of many greenhouse
gases in the atmosphere, the large heat capacity of the oceans, and the long memory of
other components of the climate system, such
as ice sheets and the biosphere— human
modifications of the climate system through
greenhouse gas emissions are likely to persist
for many centuries in the absence of appropriate mitigation measures (2).
A common response to the uncertain
risks of future climate change is to develop
climate policy as a sequence of small steps.
The Kyoto protocol, once enacted, will
commit the signatories to a nominal reduction of greenhouse gas emissions by 5%
between 2008 and 2012, relative to 1990.
The protocol is a historic first step toward
reversing the trend of continually increasing greenhouse gas emissions and will provide valuable experience in the application
of various mitigation instruments such as
tradable emission permits. However, a
nominal emission reduction of only 5% by
a subset of the world’s nations will have a
negligible impact on future global warming. To avoid major long-term climate
change, average per capita greenhouse gas
emissions must be reduced to a small fraction of the present levels of developed
countries on the time scale of a century (2).
European Climate Forum, www.european-climateforum.net. 2Max Planck Institute for Meteorology,
Hamburg, Germany. 3Institute of Marine Research,
Kiel, Germany. 4Oldenburg University, Oldenburg,
Germany. 5Chalmers University of Technology, Göteberg, Sweden. 6Potsdam Institute of Climate Impact
Research, Potsdam, Germany. 7Nansen Environmental
and Remote Sensing Center/Geophysical Institute,
University of Bergen, Bergen, Norway. 8Paul Scherrer
Institut, Eidgenössische Technische Hochschule, Zurich, Switzerland.
1
*To whom correspondence should be addressed. Email: klaus.hasselmann@dkrz.de
Such reductions cannot be achieved by simply extrapolating short-term policies but
require a broader spectrum of instruments.
Most investigations (2–4) and public attention have focused on the projected climate
change in this century. A potentially far more
serious problem, however, is the global
warming anticipated in subsequent centuries
if greenhouse gas emissions continue to increase unabated (Fig. 1, left panels) (5–7).
The projected temperature and sea level
changes for the next millennium greatly exceed those in the next hundred years (Fig. 1,
yellow boxes). If all estimated fossil fuel
resources are burnt, CO2 concentrations between 1200 parts per million (ppm) (scenario
C in Fig. 1) and 4000 ppm (scenario E in Fig.
1) are predicted in the second half of this
millennium, leading to temperature increases
of 4°C to 9°C and a sea level rise of 3 to 8 m.
Predictions of this magnitude are beyond the
calibration ranges of climate models and
must therefore be treated with caution (8).
However, the predicted climate change clearly far exceeds the natural climate variability
(⬃1°C to 2°C) experienced in the past 10,000
years. Even if emissions are frozen at present
levels, the accumulated emissions over several centuries still yield climate change on the
order of the lower business-as-usual (BAU)
scenario C.
Major climate change can be avoided in
the long term only by reducing global emissions to a small fraction of present levels
within one or two centuries. As an example,
we have computed optimal CO2 emissions
paths that minimize the time-integrated sum
of climate damage and mitigation costs, using
an integrated assessment model consisting of
a nonlinear impulse response climate model
(7) coupled to an elementary economic model (9) (Fig. 1, right panels). Cost-benefit analyses depend on many controversial assumptions, such as the role of economic inertia
(included in case a, ignored in case b), the
impact of declining costs for new technologies, and the discount factors applied to future climate change mitigation and adaptation
costs (10–14). However, the resultant longterm climate change is insensitive to the details of the optimal emission path (compare
curves a and b), provided the emissions are
sufficiently reduced. Because of the long residence time of CO2 in the atmosphere (⬎100
years), the climatic response is governed by
the cumulative CO2 emissions rather than by
the detailed path.
The impact of the Kyoto agreement (k in
Fig. 1, right panels) is hardly discernible on
the millennial time scale, suggesting that the
Kyoto debate should focus on the long-term
implications of the protocol rather than on its
short-term effectiveness. The Kyoto targets
may not be met by some countries and may
be exceeded by others. Important in either
case is that the Kyoto policy is accompanied
by measures that ensure continuing reductions in subsequent decades.
Because of the 10-year horizon of the
Kyoto protocol, climate policy has tended to
focus on promoting mitigation technologies
that are currently most cost-effective, such as
wind energy, biomass fuels, fuel switching
from coal and oil to gas, and improved energy
efficiency in transportation, buildings, and
industry. In the short to medium term, the
combined mitigation potential of these technologies is substantial: It has been estimated
that, if fully implemented, they could halve
global greenhouse gas emissions relative to
the BAU level within two decades (4). The
market-based instruments (such as tradable
emission permits and tax incentives) used to
meet the more modest 5% Kyoto reduction
targets will accelerate the penetration of these
technologies into the marketplace but will be
inadequate to realize the full potential of
these technologies.
Yet, even if forcefully implemented, currently available low-cost technologies have limited capacity for substantial global emission
reduction and will not be able to counter the
rising emissions projected for the long term.
Future emissions will be driven mainly by the
expanding populations of the developing world,
which strive to achieve the same living standards as the industrial countries. An emissions
reduction of 50% applied to a projected BAU
increase in this century by a factor of four (2–4)
still leads to a doubling of emissions, far from
the long-term target of near-zero emissions.
Furthermore, the mitigation costs for today’s
technologies are estimated to rise rapidly if per
capita emissions are reduced by more than half
(4). Thus, although the Kyoto protocol will
www.sciencemag.org SCIENCE VOL 302 12 DECEMBER 2003
Downloaded from www.sciencemag.org on January 11, 2008
The Challenge of Long-Term Climate Change
SPECIAL SECTION
VIEWPOINT
1923
boost technologies that are cost-effective in the
alone will not motivate businesses and the
from ⬃1 to 3% of gross domestic product
short term, further emission reductions in the
public to change established practices and
(GDP) (4 ), similar to the annual GDP
post-Kyoto period could be limited by prohibbehavioral patterns. The goal of long-term
growth rate in many countries. Thus, imitive costs. Without affordable new technoloclimate policy must be to influence busiplementation of an effective climate policy
gies capable of higher global emission reducness investments, research, education, and
over a time period of, say, 50 years would
tions, stricter emission reduction targets will be
public perceptions such that stringent emisdelay economic growth by only about a
considered impossible to meet and will not
sion-reduction targets—although not atyear over the same period (26 ). This apbe adopted.
tainable today— become acceptable at a
pears to be an acceptable price for avoiding
Although no such technology is yet ecolater time.
the risks of climate change. However, benomically competitive, there exist many promAlthough major changes are necessary,
cause the global political-economic system
ising candidates (15, 16), ranging from solar
the long time scales of the climate system
exhibits considerable inertia, a transition to a
thermal or photovoltaic energy—in combinaallow a gradual transition (24, 25). Estimatsustainable climate can be achieved without
tion with hydrogen technology—to carbon seed costs to halve global emissions range
major socioeconomic dislocations only if the
questration in geological
introduction of appropriate
formations or the ocean
measures addressing the
(17–20), advanced nuclear
long-term mitigation goals
fission, and nuclear fusion
is not delayed.
(4, 15, 16). Which technolScience can assist the
ogy, or mix of technologies,
development of long-term
will ultimately prove most
climate policies by providcost-effective cannot be preing detailed analyses of the
dicted. We will need to actechnological options and
cept these uncertainties and
their implications for nasupport a number of compettional economies and global
ing technologies in order to
development. The Intergovhave available several comernmental Panel on Climate
mercially viable alternatives
Change (IPCC) has played a
when the large-scale transipivotal role in the climate
tion to low-emission technoldebate by presenting authorogies becomes more urgent.
itative reviews of the state
Although short-term cliof science and on climate
mate policy can be formuchange impact, mitigation,
lated in terms of emission
and policy. Similar expertargets and implemented
tise should be made availwith instruments that interable to climate negotiators
nalize the costs incurred by
in the form of timely analyclimate change (“polluterses of the implications of
pays principle”), long-term
alternative climate policy
climate policy will require a
regimes for the individual
broader spectrum of measignatories of the United
sures extending well beNations Framework Conyond the traditional horivention on Climate Change.
zon of government policies
Although binding long-term
or business investment decommitments cannot be excisions. The entry of new
pected from governments,
technologies into the
declarations of long-term
marketplace depends on
policy goals and visible acmultiple incentives and
tions to achieve these goals
feedbacks, including priare essential for the investvate investments; government plans of businesses,
ment investments in
particularly for energy techinfrastructure and subsinologies characterized by
dies for pilot plants; prolong capital lifetimes. A
tected niche markets; and
long-term perspective is
changes in consumer
equally important for the
preferences and life- Fig. 1. CO emissions and concentrations, global mean near surface temperature, and global public, who must understyles (21–23). Climate is mean sea2 level for business-as-usual (BAU) emission scenarios (left) and optimized stand and support the polia public good that de- cost/benefit (C/B) trajectories (right; note change of scale). The BAU scenarios assume that cies. Binding commitments
mands communal action all fossil fuel resources, ranging from 4000 gigatons of carbon (GtC) (conventional resourc- to meet short-term emisfor its protection, includ- es, C) to 15,000 GtC (conventional plus exotic resources, E), are used. The sea level rise sion-reduction targets must
ing the involvement of represents the sum of thermal expansion of the warming ocean, the melting of smaller therefore go hand in hand
inland glaciers, and the slow melting of the Greenland Ice Sheet (1). Inclusion of other
citizens and institutions greenhouse gases could increase the peak values by ⬃10 to 20%. The cost/benefit solutions with clearly defined stratesuch as the media that include (a) or ignore (b) economic inertia. Pronounced differences between these cases in gies to achieve substantially
shape long-term public the short term have little impact on long-term climate. The impact of the Kyoto period (k) more stringent reductions in
attitudes. Self-interest is not discernible on these multicentennial time scales.
the longer term.
1924
12 DECEMBER 2003 VOL 302 SCIENCE www.sciencemag.org
Downloaded from www.sciencemag.org on January 11, 2008
SPECIAL SECTION
TRAGEDY OF THE COMMONS?
TRAGEDY OF THE COMMONS?
16.
17.
18.
19.
20.
21.
22.
23. Examples are tradable renewable energy permits (27) and
long-term policies in tradable emission permits (28, 29).
24. J. Alcamo, E. Kreileman, Global Environ. Change 6,
305 (1996).
25. B. C. O’Neill, M. Oppenheimer, Science 296, 1971 (2002).
26. C. Azar, S. H. Schneider, Ecol. Econ. 42, 73 (2002).
27. D. Barry, Ecol. Econ. 42, 369 (2002).
28. S. C. Peck, T. J. Teisberg, in Risk and Uncertainty in
Environmental and Resource Economics, J. Wesseler,
H-P Weikard, Eds. (Edward Elgar, United Kingdom, in
press), chap. 9.
29. M. Leimbach, Energy Policy 31, 1033 (2003).
30. The views expressed in this article evolved from discussions with members and guests of the European Climate Forum (ECF). We acknowledge constructive comments from G. Berz, C. Carraro, B. Eliasson, J. Engelhard,
J. Gretz, B. Hare, J.-C. Hourcade, M. Hulme, M. Mcfarland, N. Otter, H.-J. Schellnhuber, S. Singer, and S. C.
Peck. However, ECF does not endorse specific views
expressed by its members, and this article does not
represent an ECF consensus view.
Web Resources
www.sciencemag.org/cgi/content/full/302/5652/1923/
DC1
VIEWPOINT
Climate Change: The Political Situation
Robert T. Watson
Human-induced climate change is one of the
most important environmental issues facing
society worldwide. The overwhelming majority of scientific experts and governments
acknowledge that there is strong scientific
evidence demonstrating that human activities
are changing the Earth’s climate and that
further human-induced climate change is inevitable. Changes in the Earth’s climate are
projected to adversely affect socioeconomic
systems (such as water, agriculture, forestry,
and fisheries), terrestrial and aquatic ecological systems, and human health. Developing
countries are projected to be most adversely
affected, and poor people within them are the
most vulnerable. The magnitude and timing
of changes in the Earth’s climate will depend
on the future demand for energy, the way it is
produced and used, and changes in land use,
which in turn affect emissions of greenhouse
gases and aerosol precursors.
The most comprehensive and ambitious attempt to negotiate binding limits on greenhouse
gas emissions is contained in the 1997 Kyoto
Protocol, an agreement forged in a meeting of
more than 160 nations, in which most developed countries agreed to reduce their emissions
by 5 to 10% relative to the levels emitted in
1990. Although the near-term challenge for
most industrialized countries is to achieve their
Kyoto targets, the long-term challenge is to
meet the objectives of Article 2 of the United
World Bank, 1818 H Street N.W., Washington, DC
20433, USA. E-mail: rwatson@worldbank.org
Nations Framework Convention on Climate
Change (UNFCCC), i.e., stabilization of greenhouse gas concentrations in the atmosphere at
levels that would prevent dangerous anthropogenic interference with the climate system,
with specific attention being paid to food security, ecological systems, and sustainable economic development. To stabilize the atmospheric concentration of carbon dioxide
requires that emissions eventually be reduced
to only a small fraction of current emissions,
i.e., 5 to 10% of current emissions.
All major industrialized countries except the
United States, the Russian Federation, and Australia have ratified the Kyoto Protocol. The
United States and Australia have publicly stated
that they will not ratify it, and statements from
the Russian Federation are contradictory. Russian ratification is essential for the Kyoto Protocol to enter into force.
The United States has stated that the Kyoto
Protocol is flawed policy for four reasons:
1) There are still considerable scientific
uncertainties. However, although it is possible that the projected human-induced changes
in climate have been overestimated, it is
equally possible that they have been underestimated. Hence, scientific uncertainties, as
agreed by the governments under Article 3 of
the UNFCCC, are no excuse for inaction (the
precautionary principle).
2) High compliance costs would hurt the
U.S. economy. This is in contrast to the analysis
of the Intergovernmental Panel on Climate
Change (IPCC), which estimated that the costs
of compliance for the United States would be
between US$14 and US$135 per ton of carbon
avoided with international carbon dioxide emissions trading (a 5-cents-per-gallon gasoline tax
would be equivalent to US$20 per ton of carbon). These costs could be further reduced by the
use of carbon sinks, by carbon trading with
developing countries, and by the reduction of
other greenhouse gas emissions.
3) It is not fair, because large developing
countries such as India and China are not
obligated to reduce their emissions. However,
fairness is an equity issue. The parties to the
Kyoto Protocol agreed that industrialized
countries had an obligation to take the first
steps to reduce their greenhouse gas emissions, recognizing that ⬃80% of the total
anthropogenic emissions of greenhouse gases
have been emitted from industrialized countries (the United States currently emits ⬃25%
of global emissions); that per capita emissions in industrialized countries far exceed
those from developing countries; that developing countries do not have the financial,
technological, or institutional capability of
industrialized countries to address the issue;
and that increased use of energy is essential
for poverty alleviation and long-term economic growth in developing countries.
4) It will not be effective, because developing countries are not obligated to reduce
their emissions. It is true that long-term stabilization of the atmospheric concentration of
greenhouse gases cannot be achieved without
global reductions, especially given that most
www.sciencemag.org SCIENCE VOL 302 12 DECEMBER 2003
Downloaded from www.sciencemag.org on January 11, 2008
9.
10.
11.
12.
13.
14.
15.
Gulf Stream and deep ocean circulation system, a
break-off of the West Antarctic ice sheet, or the
release of methane presently frozen in permafrost
regions.
K. Hasselmann et al., Climatic Change 37, 345 (1997).
W. D. Nordhaus, Science 294, 1283 (2001).
P. G. Brown, Climatic Change 37, 329 (1997).
G. Heal, Climatic Change 37, 335 (1997).
W. D. Nordhaus, Climatic Change 37, 315 (1997).
K. Hasselmann, Climatic Change 41, 333 (1999).
J. Goldemberg, Ed. World Energy Assessment
(2000): Energy and the Challenge of Sustainability
(United Nations Development Programme, United
Nations Department of Economics and Social Affairs, World Energy Council, New York, 2001).
M. I. Hoffert et al., Science 298, 981 (2002).
B. P. Eliassen et al., Eds., Greenhouse Gas Control
Technologies (Pergamon, Amsterdam, 1999).
K. S. Lackner, Science 300, 167 (2003).
P. G. Brewer et al., Science 284, 943 (1999).
H. Drange et al., Geophys. Res. Lett. 28, 2637
(2001).
M. Haduong et al., Nature 389, 270 (1997).
O. Edenhofer et al., in preparation.
SPECIAL SECTION
References and Notes
1. T. P. Barnett et al., Bull. Am. Meteor. Soc. 80, 2631
(1999).
2. Intergovernmental Panel on Climate Change, Climate
Change 2001, Working Group 1: The Scientific Basis,
J. T. Houghton et al., Eds. (Cambridge University
Press, 2001).
3. IPCC, Climate Change 2001, Working Group 2:
Impacts, Adaptation, and Vulnerability, J. J. McCarthy et al., Eds. (Cambridge University Press,
2001).
4. IPCC, Climate Change 2001, Working Group 3: Mitigation, B. Metz et al., Eds. (Cambridge University
Press, 2001).
5. W. R. Cline, The Economics of Global Warming (Institute for International Economics, Washington, DC,
1992).
6. T. J. Crowley, K-Y Kim, Geophys. Res. Lett. 22, 933
(1995).
7. G. R. Hooss et al., Clim. Dyn. 18, 189 (2001).
8. The uncertainties of climate predictions are estimated to be ⬃50%, excluding instabilities of the
climate system that could yield substantially larger
changes, for example, through the collapse of the
1925